This invention relates to an electronic constant speed control for a hydrostatic transmission having an output shaft driven by a hydraulic motor in either direction of rotation thereof and at any speed between zero and a maximum speed, with both open loop and closed loop control of the position of the displacement controlling member of the pump of the hydrostatic transmission.
Electronic constant speed controls for hydrostatic transmissions are known in the prior art. Such controls include those marketed by the assignee of this application, as well as a control offered by a company for the power take-off drive on a large, articulated farm tractor. Additional prior art includes Hawkins U.S. Pat. No. 2,939,285 and Herndon U.S. Pat. No. 3,529,422.
The electronic constant speed control disclosed herein distinguishes from the prior art in having a full range of speed control in either direction of rotation of an output shaft driven by the transmission with both open and closed loop control, with the capability of operating under an open loop control only and with control circuit features optimizing steady state accuracy and transient response, reduction of errors occurring during speed changes of the pump of the hydrostatic transmission, and variable gain for the hydrostatic transmission motor speed feedback circuit to enable accurate operation at very low speeds of said motor. The control includes a provision for operating in a manual open loop mode with automatic switching to said manual mode upon failure of the closed loop portion of the circuit, with prevention of undesirable transients in an error integrator of the closed loop part of the circuit upon switching the electronic control from the manual to automatic mode.